Background

The Sengbusch Classification System

The Sengbusch Classification system defines five degrees of monoecious forms. The methods developed by R. von Sengbusch and H. Neuer (1943) are the foundation of breeding technology for monoecious hemp:

Genetic factors dispose a plant to become male or female whereas environmental factors including the light cycle can alter the sexual expression. The light-dark cycle determines when hemp begins to flower (males appear).

A hemp plant in the vegetative growth phase requires more than 12–13 hours of light per day to stay vegetative. Flowering usually occurs when darkness equals at least 12 hours per day. The flowering cycle can last for weeks, depending on the variety and environmental conditions.

Regardless of seeding date, hemp will generally start to flower about the same time every year (photoperiod). Planting hemp early in the spring can produce a taller more robust plant. Late seeding of hemp can potentially produce shorter plants, but both seeding dates will result in flowering about the same time every year. In Canada, hemp males usually appear about the middle of July (within a week).

As the growing season progresses, the bottom leaves die (atrophy) due to the lack of sunlight that can reach the bottom of the crop. Male plants die soon after shedding pollen.